I. Technical Field
The present invention relates to automated circuit board inspection systems and techniques. More specifically, the present invention relates to a novel method and apparatus for performing measurements of the structural characteristics of a manufactured circuit board having solder joints thereupon by a fully automatic real-time digital X-ray radiographic inspection techniques.
II. Background Art
In electronics, components are typically mounted upon or inserted into a circuit board. The electrical contact between the circuit board and the components is assured by soldering of the component into permanent position. Thereafter, the electrical integrity of the circuit board depends upon the mechanical integrity of the soldering completed during the circuit board assembly. Soldering processes are well-known and may be reasonably controlled to correct solder related deficiencies. However, soldering processes do not always work perfectly with deficiencies such as solder skips, bridges, insufficient amounts of solder, blow-holes and pin holes which can occur as a result of variations in materials in the solder process. Defects, such as those just mentioned, occur sufficiently often such that it is mandatory to inspect solder connections to reduce solder connection related failures.
Traditionally, solder quality inspection has been performed visually merely because of the fact that humans sense more data visually than with any other of the senses. As a result, previous inspections standards for solder quality were written in terms of the external appearance of the solder connection. The objective of a solder quality inspection is also to insure mechanical integrity of the solder connection. Since mechanical integrity is dependent upon the interior structure of the solder connection, visual inspection techniques are wholly deficient in verifying mechanical integrity.
The mechanical integrity of a solder connection depends upon the type of solder alloy used, the solder connection structure (surface mount versus pin-through-hole) and the presence of an adequate and uniform volume of solder bonding (or wetting) of the electronic component to the circuit board. Visual inspection is regarded as a qualitative test, rather than a quantitative test. In visual inspections, the external appearance of the solder connection is used to infer internal structural integrity. Visual inspections are an accepted solder quality inspection practice used to indentify gross variations in connection structures, such as missing pins, insufficient solder volume, excess solder or bridging. However, visual inspection cannot verify the uniformity of the solder within the connection, and cannot detect defects that are hidden below components mounted on the circuit board. Solder uniformity has a critical influence on the strength and durability of the solder connection. Solder connection strength and uniformity are particularly important in the connection of surface mount devices where the devices are held to the circuit board by the solder connection. It is well-known in the surface mount device art that solder connections are more susceptible to thermal and mechanical stress related failures than pin mounted devices. In solder mounted devices, visually inspected structurally marginal connections, due to solder non-uniformity, may still provide electrical connection without the defect being discovered in stress testing. As a result, the marginal connection or hidden defect is a likely candidate for a long term failure while under normal mechanical and thermal stress. With a greater number of surface mount components being used in circuit boards, visual inspections are proving, in many cases, to be deficient in detecting structural deficiencies in the solder connections.
Solder quality visual inspection systems examine the circuit boards to detect defects such as components missing; components incorrectly oriented; missing or bent component pins or leads such that the component does not make a connection; cracked solder connections; solder bridge between component pins or circuit board pad; small holes present at the surface of the connection; insufficient clearance between component pins; excess solder in the connection; insufficient solder in the connection; solder spurs, spikes, balls or splashes; poor solder wetting on the board or the component; a misshaped solder connection which indicates surface tension problems; component askew pads on the circuit board; component pins lifted or tilted from the circuit board; component pins misaligned with circuit board pads; and component pins not projecting through the circuit board hole. Each of the above defects indicate conditions that can compromise the electrical and mechanical integrity of the circuit board.
In many applications, defects are hidden from the human eye or machine vision inspection systems. An example of such a defect is in the case of solder porosity or voids. While defects may not be masked by visual barrier, increasing circuit density may result in defects which are not readily apparent to the human eye at production line rates. With machine vision inspection systems, inspection deficiencies still exist. For example, machine vision inspection systems would be unavailable for inspecting defects such as solder balls under a pin grid array.
Typical inspection systems are oriented toward finding defects rather than avoiding the defects in the production of future items. The avoidance of defects essentially requires rapid process control feedback from the inspection system to the production line. Process control feedback of the defects requires quantitative analysis feedback of the deficiencies and providing this information to the production line to control the soldering process. For example, quantitative quality data such as the excess amount of solder volume present in a solder connection must be fed back to the soldering process to reduce the solder used in future units so as to eliminate the defect. The present human and machine vision inspection systems lack the ability to provide quantitative data for feedback control to the process lines for correcting process deficiencies. With faster production lines, inspection systems must detect process drift before the production line turns out numerous defective items. For solder quality process control, the inspection accuracy and repeatability needed must detect even the smallest changes in solder connection before they grow to become defects.